Can I assign multiple users to approve of a process?

Yes, you can. In fact, you have two options.

1. All assigned users should approve of a document/process. In this case you need parallel approval without parallel branches.
2. At least one user should approve of a document/process. In this case there are three options:
   1. Create a Rule that describes the roles of these people and attach it to a single Transition.
   2. Create separate Rules and unite them in a single Transition with the OR clause.
   3. Create several Transitions, one for each Rule.

Learn more about rules and roles. Besides, Workflow engine supports parallel branching by using subprocesses.